MSP-EXP430FR5994: Powering the device from the DAC121S101EVM

Hi Karthik,

Thanks for using MSP devices. I will check with our experts for your question and reply to you.

Hi Karthik,
the critical point about the super cap charging is the current limitation. As described in the User's Guide for the MSP-EXP430FR5994 the recommended jumper J8 setting, is to set it to charge through the current limiting resistor. Otherwise the super cap is basically a short for the charging source, in your case the DAC. The current drive capability of your DAC seems to be pretty limited with 50mA max. (I assume, you've indicated >50mA, but it's probably rather <50mA). This means there will be repetitive current peaks exceeding the DAC specification by far. This means the DAC output might get damaged completely, or it's performance might degrade. So it would be safer using a current limiting resistor in series to the output of the DAC, to limit the current.

It also sounds to me from your description, that you want to use the DAC to turn the supply for the MSP430 on and off very quickly and repeatedly. To shut down the MSP430 supply, means, you'd need to discharge the super cap and other capacitors on the PCB. This would again load the DAC with high current sinking loads, exceeding the 50mA value.

So from my point of view, especially, when you intend to work with the super cap, you will need a more complex setup, e.g. an arbitrary power supply, which allows you to control the current limits, rather than this simple DAC approach, at least, when you want to go with a safe and reliable setup.

Best regards
Peter

Hi Peter,

Thank you for your detailed response! Yes, the current would be quite high to and from the DAC. you mentioned a current limiting resistor. Is that something already provided on the board? It would actually be preferable for me if the current were lower than the max (which as you say is also likely to damage the DAC). If one isnt available on the MSP board itself would you be to recommend one to me? Also, should i pair it with a diode maybe to avoid current sinking via the DAC during the discharge phase? 

Regarding speed, I am still only planning to change the outputs at KHz not MHz speeds. 

Thank you,

Karthik 

Hi Karthik,
in terms of the limiting resistor, yes there is one provided on the PCB. It is can be selected by the respective J8 setting. In one position it's connecting the super cap directly to the MSP430 supply, in the other, for charging, it is connecting the super cap with the MSP430 supply through this resistor. This is described in the section 2.3.3.1 Charging the Super Cap of the Launchpad User's Guide slau678a.
According to the design files, it is 22Ohm. This gives in worst case still 150mA, in case the super cap is completely discharged.

To be able to answer your question on the diode or the external circuitry more precisely, I would need to get a better understanding, what you're actually trying to do. From your initial post, I was understanding, you'd like to switch the power supply of the MSP430 automatically on/off by the DAC. That's why I did not come up with the insertion of a diode, as this would of course make an active shut down of the supply voltage impossible, and dependent on the operating mode of the MSP430, it might even take minutes or more, to shut down the MSP430, as it would be powered from the super cap. So please be more specific. 1kHz e.g. means, you're turning the supply voltage on/of every ms. That would require extremely fast charge discharge of the super cap, requiring high current supply and sinking.

Best regards
Peter

Sorry i was unclear in my original post. The 1Khz refered to the rate of changing my DAC only. In fact I want the super cap to charge and discharge naturally. 

So to explain my situation a bit more, I am trying to mimic an energy harvesting system. So i want to provide energy to the supercap for a short period of time. I want this provided energy to be small (in keeping with energy harvesting sources. so maybe 1-5 mA maybe). Once the super cap has reached a threshold (maybe 100mF worth of charge), then let the system run off of that energy. once the systems uses all the stored energy it should shut down. all the while the charging would continue slowly charging up the cap. then once again when the cap has 100mF worth of charge, the system would run again. 

The reason I need the DAC is to provide the 1-5mA charging current. the reson I need a DAC is i would like to input a 'real' voltage trace (that i collected using a Wifi device). So it would act as though the device is actually being charged from a wifi source while running.  

Why not be safe and put a higher current buffer on the DAC? A beefier op-amp or an emitter follower might work fine.

Any suggestions for those? In fact i dont want to drive too high a current. so if the DAC can drive 1-5mA (even with a current limiting resistor) that would work for me.